380. Insert Delete GetRandom O(1)(unsolved)

随机集合实现

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本文介绍了一个支持插入、删除和随机返回元素的数据结构实现。该结构能在平均常数时间内完成操作，并保持每个元素被随机返回的概率相等。使用了哈希表和动态数组结合的方式，确保高效的操作执行。

Design a data structure that supports all following operations in average O(1) time.

insert(val): Inserts an item val to the set if not already present.
remove(val): Removes an item val from the set if present.
getRandom: Returns a random element from current set of elements. Each element must have the same probability of being returned.
Example:

// Init an empty set.
RandomizedSet randomSet = new RandomizedSet();

// Inserts 1 to the set. Returns true as 1 was inserted successfully.
randomSet.insert(1);

// Returns false as 2 does not exist in the set.
randomSet.remove(2);

// Inserts 2 to the set, returns true. Set now contains [1,2].
randomSet.insert(2);

// getRandom should return either 1 or 2 randomly.
randomSet.getRandom();

// Removes 1 from the set, returns true. Set now contains [2].
randomSet.remove(1);

// 2 was already in the set, so return false.
randomSet.insert(2);

// Since 2 is the only number in the set, getRandom always return 2.
randomSet.getRandom();

解答：
1.rand()%nums.size() 是取随机数方法，从0到noms.size()-1;
2.设一个map和一个vector的原因是这样可以很方便地取随机数。map里存的是nums里val的位置。nums存的是所有val的取值。

class RandomizedSet {
public:
    /** Initialize your data structure here. */
    RandomizedSet() {

    }

    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    bool insert(int val) {
        if(m.find(val)!=m.end()) return false;
        nums.push_back(val);
        m[val]=nums.size()-1 ;
        return true;
    }

    /** Removes a value from the set. Returns true if the set contained the specified element. */
    bool remove(int val) {
        if(m.find(val)==m.end()) return false;
        int last=nums.back();
        nums[m[val]]=last;
        m[last]=m[val];
        nums.pop_back();
        m.erase(val);
        return true;
    }

    /** Get a random element from the set. */
    int getRandom() {
        return nums[rand()%nums.size()];
    }

private:
    vector<int> nums;
    map<int,int> m;

};

/**
 * Your RandomizedSet object will be instantiated and called as such:
 * RandomizedSet obj = new RandomizedSet();
 * bool param_1 = obj.insert(val);
 * bool param_2 = obj.remove(val);
 * int param_3 = obj.getRandom();
 */